1. Field of the Invention
The present invention relates to oriented multilayer films. More particularly, the invention pertains to coextruded or laminated films having at least one layer of a fluoropolymer such as poly(chlorotrifluoro ethylene) (PCTFE) homopolymer or copolymer, a layer of a thermoplastic homopolymer or copolymer and an intermediate adhesive layer which is preferably a polyolefin having at least one functional moiety of an unsaturated carboxylic acid and/or anhydride thereof.
2. Description of the Prior Art
It is well known in the art to produce oriented polymeric films. See, e.g. U.S. Pat. No. 4,011,874. However, such films tend to expand in the direction perpendicular to the direction of stretching.
It is also known in the art to produce single layer and multilayer fluoropolymer films. See, e.g. U.S. Pat. Nos. 4,677,017; 4,659,625 and 5,139,878. As shown in U.S. Pat. No. 4,011,874, polymers may be melt extruded through an orifice, and the molten polymer quickly quenched and then drawn. Although the moisture and vapor barrier properties of oriented fluoropolymer film has been known for years, successful film orientation has been exceedingly difficult due to technical difficulties during the casting and orientation processes. Such films tend to expand in the direction perpendicular to the direction of stretching. PCTFE is exceptionally difficult to orient due to its extremely fast crystallization rate and thermally induced self-orientation. Its fast crystallization rate produces a highly crystalline structure that hinders orientation and actually prevents further orientation beyond a certain point. Its thermally induced self-orientation results in a film which, upon unconstrained heating, self extends in the machine or longitudinally stretched direction and shrinks in the transverse direction.
Most earlier attempts to stretch PCTFE films have failed either due to its high degree of film crystallinity, nonuniform crystallinity, self-orientation or a combination of these factors. Prior art studies of the orientation of PCTFE homopolymer report a limit of a three to four times orientation or stretch ratio in either the machine direction (MD) or transverse direction (TD). For example, U.S. Pat. No. 4,544,721 describes a substantially amorphous chlorotrifluoroethylene polymer monolayer film which is oriented at least 2.5 times its original length, but no more than five times in the MD. It also discloses therein that attempts to stretch crystalline PCTFE result in films that contain holes or tears, or which are uneven in thickness. Other known attempts to stretch PCTFE homopolymer more than five times its unstretched length result in film fibrilation and ultimate breakage. See, e.g. U.S. Pat. No. 4,510,301 (orients film containing a copolymer of 40 to 60 mole percent ethylene and chlorotrifluoroethylene). U.S. Pat. No. 4,519,969 discloses a biaxially stretched film and a method for the manufacture thereof, containing at 90 mole % of ethylene-tetrafluoroethylene copolymer having a specific crystallization property. Various attempts have also been made to produce a multilayer fluoropolymer film structure, with most emphasis focused on the selection of the adhesive materials. U.S. Pat. No. 4,677,017 discloses coextruded multilayer films which include at least one fluoropolymer and at least one thermoplastic film which are jointed by the use of an adhesive polymer, particularly ethylene/vinyl acetate resins. U.S. Pat. No. 4,659,625 discloses a fluoropolymer multilayer film structure which utilizes a vinyl acetate polymer as an adhesive layer. U.S. Pat. No. 5,139,878, which is incorporated herein by reference, discloses a fluoropolymer film structure using an adhesive layer selected from the group consisting of alkyl ester copolymers of an olefin, the modified polyolefins, and their blends. U.S. Pat. No. 3,355,347 relates to a laminated film of poly(chlorotrifluoroethylene) and plasticized poly(vinyl chloride) using a polymerized ethylene imine.
It would be desirable to produce a much more highly oriented, dimensionally stable fluoropolymer film since as the degree of attainable orientation is increased, the properties of mechanical strength, toughness, and water vapor barrier capability are significantly improved without increasing the film gauge. It would also be desirable to produce a multilayered film structure which is dimensionally stable and uniform across its entire width. Through coextrusion of fluoropolymer films with another polymer or polymers it is now possible that fluoropolymer films can be easily biaxially oriented. Not only can fluoropolymers be stretched uniaxially in either the longitudinal or transverse directions, but such films can also be biaxially oriented simultaneously or sequentially. Significant improvement in properties are observed in such biaxially oriented films, i.e., tensile modulus, mechanical properties, toughness, water vapor transmission rate and barrier properties. Achieving a high barrier fluoropolymer film through orientation would be useful for medical packaging, pharmaceutical packaging and other industrial uses.